Methods and apparatuses pertaining to the above mentioned technical field are known. US 2009/0294647 A1 (Karsten Michelmann) for example describes an ion mobility spectrometer which is coupled to a mass spectrometer and a corresponding measuring method. In this example, the sample comprises ions. A beam of these ions is modulated according to a continuous modulation function, wherein the modulation frequency of the modulation function is varied over a large frequency range. For a measurement, the modulated ion beam is passed through a drifting region, after what the ions are detected with the mass spectrometer serving as detector for the ion mobility spectrometer. In order to obtain the ion mobility spectrum, the detector signal provided by the detector is correlated with the modulation function according to which the ion beam was initially modulated.
Another ion mobility spectrometer and corresponding method are disclosed in U.S. Pat. No. 7,417,222 B1 (Sandia Corp). There as well, the ion beam is modulated according to a modulation function and the measured signal is correlated with the modulation function. But in contrast to US 2009/0294647 A1, the modulation function may also be a binary function.
These known methods and apparatuses have in common that the sample comprises ions. A beam of these ions is modulated according to a modulation function. After this modulation, the ions are passed through a drifting region and are detected at the end of the drifting region by a detector which provides a detector signal. By calculating a correlation of the modulation function with the detector signal, the ion mobility spectrum is obtained. This procedure for obtaining the ion mobility spectra is employed because it is not required to know the starting time of each individual ion as it would be if directly measuring the ion's flight time. Consequently, it is possible to pass at the same time more than one pulse or packet of ions through the drifting region. This has the advantage that the ion mobility of more ions can be measured within the same period of time, thus enabling a higher data collection rate.
The disadvantage of these methods and apparatuses is however that the resolution of the measured spectra is limited.